brake pedal low presure
 

all of a sudden my brake pedal started acting up. when I push on the brake I can push it most of the way to the floor with no resistance then get resistance right near floor. braking performance is a fraction of what it used to be. also, use to have wobble while braking like a worn rotor, this has now gone away. I shut off the engine and pumped the brakes and got normal resistance immediately, but holding my foot on the brake while I turned on the engine, the pressure disappeared as the engine turned over. What are the likely causes?

Could be low fluid in the master cylinder allowed air to get in the lines. You'll need to bleed the brakes if that's the problem. Could be a failing power brake booster. Get the entire system checked out. It's dangerous in this condition.

Now it is working fine again. Took it to Dealer who said it was leaking rear cylinders. (but fluid reservoir is full).

was working fine fore a few weeks then started acting up again. So I opened up the reservoir and found out that the dealer had switched the diaphragm on the cap with one that was too big and cut up to fit. So I think that has been letting air into the system.

mmm i doubt that more then likely the fluid got contaminated and made the seal swell up what or why would some one take a rubber diaphragm from your car and put it on some one else ? not likely.

Not sure why it happened but the diaphragm was fine when I dropped the car off, then after I picked it up the diaphragm was as wide as the outside of the cap, was cut from the side, and had the cut ends overlapping so it could be squeezed in the cap.
I am guessing that this was allowing air into the brakes.

(That dealer also said I needed to replace my entire convertible hydraulic system at about 2K, when all it really needed was some more fluid.)

Fixing the cap to the resevoir didn't solve the problem. It looks like the brakes go soft after working them hard after a day. of driving in 97 deg air the brakes are acting soft again could thi be residual air in lines?

Brake fluid will absorb moisture from the air. Moisture can turn to steam if the brakes get hot enough. Steam can act just like air until the brakes cool down and the fluid can re-absorb the moisture. If the master cylider seal was shot, then it's likely that the fluid is contaminated, as pointed out by Tech several posts ago. The moisture can also cause rusting of the system and further contamination.
At this point, your best bet is to completely flush out the system and re-fill with new fluid, then properly bleed the system and re-establish the seal. Make sure you use the proper brake fluid.

Thanks, was getting that the contaminated fluid would corrode inner workings, didn't consider the moisture vaporizing and causing problems. That makes sense. Hey Pherb, I know what we're going to do today!

So far I drained the reservoir and it was filled with floating contaminates. Now I can't seem to get the drain on the rotor loosened!